infection of seed & transmission of seed borne pathogens
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Infection of Seed & Transmission of
Seed Borne PathogensLindsey du Toit
WSU Mount Vernon NWREC
Seed borne vs. seed transmittedSeed borne microorganisms:
- saprophytic- pathogenic- opportunistic
Seed borne microorganisms:- fungi- bacteria- viruses- nematodes
Classes of seed borne microorganisms
1.
Infected seed = primary inoculum source. If seed infection is controlled,
the disease is controlled2.
Important pathogen, but infected seed = minor source of inoculum
3.
Seedborne
microorganisms never demonstrated to cause disease
4.
Pathogens that infect seed in fields or in storage, & reduce seed quality
Attributes of seed transmitted organisms
•
Ability to gain access
to seed•
Ability to survive
commercial processes
–
Harvest, cleaning, treatment, storage•
Ability to establish on emerging seedlings–
Primary or secondary transmission
Biological factors that affect these attributes
•
Infection method(s)•
Timing of infection of seed crop
•
Nature of pathogen–
biotroph, necrotroph
•
Survival structures or states–
nematodes –
cryptobiosis, cysts
–
fungi and Chromistans
-
sclerotia, microsclerotia, oospores, chlamydospores
Ability to gain access to seed•
Active process of infection- within seed
•
Passive access–
present on seed surface
–
contaminant in seed lot
•
Affects ability to clean or treat seed
(from Maude, 1996)
Routes of active seed infectionstigmastyle
pollen tube
ovary wall(pericarp)ovule (seed)nucellusegg sac
testa(seed coat)
egg cell(embryo)micropylevascular tracefunicle(funicular scar = hilum)
A. Penetration through ovary wall
Routes of active seed infection
E.g.: Cladosporium
variabile (spinach), Botrytis spp. (onion)
C. Penetration through floral parts
E.g.: Ustilago nuda (grains)Cucumber mosaic virus
B. Systemic infection via vascular system
E.g.: Vascular wilt fungi,endophytesFrom Maude (1996)
Direct systemic infection via vascular system• Many viruses, e.g.:
• PSbMV
on pea• LMV on lettuce• AMV in alfalfa• PEBV in pea
• Some fungi, e.g.:•
Vascular wilts (Verticillium dahliae, Fusarium oxysporum)
• some downy mildews• Few bacteria, e.g.:
• X. campestris pv. campestris
• Direct connection between embryonic& endospermic tissue becomes disconnected as seed develops
• Potential for transmission affected bydegree of internal infection
Direct systemic infection of seedNeotyphodium coenophialum in seed next to the
embryo. Systemic infection of plants. Whole life cycle occurs inside tall fescue (mutualism).
K. Gwinn, UT
Seed borne Verticillium
dahliae in spinach
Non-inoculated control Inoculated with V. dahliae
Systemic infection from roots or seeddu Toit et al., 2005. Plant Dis. 89:4-11
Indirect systemic infection via stigma to embryo
• Pathogen moved from infected plants toflowers
• May follow pollen pathway to embryo sac• Examples:
•
pollen borne viruses –
LMV, CMV (some hosts)
• infected pollen may be less viable(poor fertilization)
• Nepoviruses• Loose smut fungi (Ustilago tritici) • Ergot fungi, not seed transmitted but
ergots can be mixed with seed• Lower rate of pollen vs. ovule transmission
(LMV)
Pollen tube entry into seedExamples:1.
loose smut of small grains
2.
CMV in spinach
Certification programs,host resistance,systemic fungicides
Indirect infection via flower or fruit• Weak necrotrophs:
•
Botrytis cinerea - infected petals remain attached to developing fruit
• Aggressive necrotrophs:• Attack floral parts directly•
e.g., Ascochyta pisi, Alternaria brassicicola
peduncle fruit coat(ovary) dorsal suture funicle*
seed coat
• Fleshy fruits (e.g., Solanaceae) –
seed attached to central placenta-
infect via calyx -
placenta –
funicle
–
embryo• Umbelliferae
& Liliaceae
–
flowers exposed in umbels• Seed transmission is typically discontinuous (infection outside
embryo)-
affected by intrinsic & environmental conditions
Brassica
seed
Alternaria brassicicola & A. brassicaeCorrelation of severity of pod spot with:
Seed germ = -0.89, Seed rot = +0.88du Toit & Derie, 2003. Fung. & Nem. Tests 58:V026
Leptosphaeria maculans(black leg)
Seed borne Stemphylium botryosum & Cladosporium variabile in spinach
Hernandez-Perez & du Toit. 2006. Plant Disease 90:137-145du Toit & Hernandez-Perez . 2005. Plant Disease 89:1305-1312
Botrytis scape
& flower blight of onion
B. aclada/B.alliiB. squamosaB. byssoidea
S.K. Mohan
Seed borneBotrytis aclada
& B. allii in onion
Bacterial leaf blight of carrot
Xanthomonas campestris pv. carotae
Pathogens use multiple points of access?
Acidovorax avenae subsp. citrullibacterial fruit blotch of watermelon
Possible mechanisms of seed infestation by A. avenae subsp. citrulliA. Penetration through ovary wall
C. Penetration through floral partsB. Systemic infection via
vascular system
??No evidence
Role of female watermelon blossoms in Acidovorax avenae subsp. citrulli seed infection
Walcott et al. Phytopathology 93:528-534
Seed borne pathogens:Implications for risk analysis
• Epidemiology of the pathogen- Potential for seed transmission- Conditions for disease development & spread- Ease of eradication
• In-field control measures- Resistant cultivars- Fungicides & forecasting- Crop rotation- Plant spacing, row orientation- Irrigation (system & schedule)
• Threshold(s) for seedborne
inoculum- Environmental conditions?- Resistant vs. susceptible cultivars?
• Alternative sources of inoculum-
Infested residues-
Soilborne
inoculum-
Infected adjacent or overwintering crops or weeds
Potential for seed transmission• Environmental factors
- temperature*- moisture*- light- pH- soil microflora- degree of internal infection of seed
• Genotype- host resistance, maternal vs. paternal- pathogen isolate/strain
• Inoculum
potential- amount of inoculum
& conduciveness of environment
- transmission rate- threshold(s) for seedborne
inoculum
- complex, lack of adequate field research
Seed borne inoculum
thresholdsXanthomonas campestris pv.
campestris• 1 in 30,000 seed•
Increases asymptomatically
before epidemic in field
Cylindrocladium parasiticum in peanuts
•
1 in 400 seed = VA threshold
Significance of seed borne carrot pathogensUmesh
et al., 1998. Plant Dis. 82:1271-1275. Bacterial blight in central CA.Threshold of ~104-105
CFU/g seed in semi-arid central CA.
Seed borne pathogens: ImplicationsAlternative inoculum
sources for seed borne carrot diseases
- Infested residues (Alternaria dauci in CA) (Gilbertson et al.)- Soil borne inoculum
(Alternaria radicina = 8 years)- Infected
adjacent or overwintering crops or related weed hosts(X. campestris pv. carotae in carrot seed crops in PNW)
du Toit et al., 2005. Plant Dis. 89:896-907.
Mean % seed transmission(% of infected seed planted)
Fungus Trial 1 (RH ~95%) Trial 2 (RH ~75%)C. variabile 5.1 (18.1) 0.1 (0.4)S. botryosum 9.1 (10.3) 3.3 (3.7)
Influence of relative humidity on
transmission of leaf spot fungi from spinach seed
Alternative sources of inoculum
for S. botryosum in spinach seed crops
Pleospora herbarum on surface spinach stem residuesdu Toit & Derie, 2003. Phytopathology
93:S22
0
10
20
30
40
50
60
1 10 20
1973 (dry summer)1974 (wet summer)
% Seed infected with B. allii
% B
ulbs
wit
h ne
ck r
ot
Relationship between infected seed & post-harvest neck rot in the UK
(from Maude & Presly, 1977b)Tichelaar, 1967;Maude & Presly,
1977aEllerbrock
& Lorbeer
(1977): Field infection sources were more important in NY
than infected seed.
Influence of environment on seed transmission ofBotrytis aclada/B. allii in onion
0
10
20
30
40
50
60
1973 1974 1975 1977 1978 1979
Treated with benomylor benomyl + thiramNon-treated
Control of neck rot in stored onion bulbs in the UK using seed treatments(from Maude, 1983. Seed Sci. & Technol. 11:829-834)
% B
ulbs
wit
h ne
ck r
ot in
sto
rage
Influence of environment on seed transmissionof Botrytis aclada/B. allii in onion:
2004 trials in WA & NY
Lot
Seedborne
Botrytis (%)
Stand count/
2 m bed(6 May)
% Plants infected with Botrytis spp.
(latent)% Neck rot
(Feb/Mar 2005)SeedCo.
WSUlab*
WA(19 Jul)
NY(20 Jul)
WA(7 Feb)
NY(15 Mar)
A 0.0 6.0 a 105 a 96.5 a 73.3 a 3.7 a 9.1 aB 14.6 16.3 b 92 a 96.6 a 74.1 a 4.0 a 0.0 aC 20.6 33.6 c 93 a 100.0 a 54.6 a 3.7 a 16.8 aD 1.9 55.3 d 92 a 98.8 a 72.6 a 2.8 a 13.5 aE 41.1 61.3 d 90 a 98.8 a 69.5 a 3.1 a 21.8 a
* = % surface-contamination detected at WA State Univ. (% ‘internal’ infection detected for lots A, B, C, D, & E
=
0, 2, 3, 7, & 6%,
respectively).
Similar results in WA in 2002 & 2003
Alternative sources of inoculum
of Botrytis for infection of onion bulb & seed crops
• Physical -
kill pathogens, not seed
• Chemical–
protect seed/seedling against pathogens
• Biological–
protect seed/seedling against pathogens
-
induce systemic resistance-
improve plant growth
Management of seed borne pathogens: Seed treatments
- Hot water- Chlorine- Aerated steam- Hot, dry air- Fungicides
-
conventional, organic, biological-
Efficacy, potential phytotoxicity:
- infected vs. infested seed- volume of seed treated- parameters- pathogens, cultivars, seed quality- drying seed- shelf-life
Management of seed borne pathogens: Seed treatments
Hot water seed treatment for spinach leaf spot
0
10
20
30
40
Inci
denc
e (%
) of s
eed
with
Cla
dosp
oriu
m v
aria
bile
0
1
2
3
0102030405060708090
100
Seed
ger
min
atio
n (%
)
40 C 45 C50 C55 C60 C
0
10
20
30
40
50
0
1
2
3
4
5
6
7
0 10 20 30 40
Duration (min)
Inci
denc
e (%
) of s
eed
with
Ste
mph
yliu
m
botr
yosu
m
0
10
20
30
40
50
60
70
80
0 10 20 30 40
Duration (min)
Trial 1 Trial 2
(du Toit & Hernandez-
Perez. 2005. Plant Disease 89:1305-1312)
0
10
20
3040
50
60
70
80
90
100
Cont
rol
Sere
nade
ASO
Mer
tect
340F
Tops
in-M
70W
PDy
nast
y 10
0FS
Thira
m42
-SM
axim
4FS
Pris
tine
XCF
Rovr
al4F
Natu
ral I
I
Trial 1Trial 2
Evaluation of fungicide seed treatments for control of seed borne Stemphylium botryosum
(du Toit et al., 2007. Plant Dis. Mgmt Reports 1:ST003)
% S
eedb
orne
S. b
otry
osum
a a
a
bb b
a
cd c d
w w w w
x
y
x
z
z
z
Exp #1
Exp #2Kodiak
Micro 108
Mycostop Mix
Natural II
Natural X
PGPR Galaxy
Subtilex
T-22 Plntrbox
Yield ShieldApron
NonTrt
Mertect
Per
cent
age
Ver
ticill
ium
0
10
20
30
40
50
60
Evaluation of organic seed treatments for spinachEvaluation of organic seed treatments for spinach (Cummings, 2007)
= Significantly less seed borne Verticillium than non-treated seed
Cmpst TExp #1
Exp #2Kodiak
Micro108
Mycostop
Natural II
Natural XPGPR
Prestop
Soilgard
SubtilexT-22
YldShldApron
Ntrt Inoc
Ntrt NonInoc
AU
DP
Cto
tal
0
500
1000
1500
2000
2500
3000
Greenhouse evaluation of organic seed/drench Greenhouse evaluation of organic seed/drench treatments for SOIL BORNE treatments for SOIL BORNE PythiumPythium ultimumultimum
(Cummings, 2007)
= Significantly less disease than non-treated seed= Significantly more disease than non-treated seed
Challenges presented by seed borne pathogens
•
Determine
definitively
if a seed borne pathogen is seed borne & seed transmitted
•
Develop thresholds
for seed borne inoculum, applicable to a range of environments
•
Regulatory implications
for seed transmitted vs. seed borne status
•
Management
of diversity of seed borne pathogens under diverse production environments
Thank you!
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